Method for processing of steel strip continuously



l1. DUNLEVY ETAL April 14, 1953 METHOD FOR PROCESSING OF' STEEL STRIPCONTINUOUSLY Filed Sept. 5

Patented Apr. 14, 1953 UITED STATE TNT OFFICE METHOD FOR PROCESSENG FSTEEL STRIP CONTINUOUSLY Application September 5, 1950, Serial No.183,214

1 Claim.

This application relates to method for cleaning metal strip continuouslyand generally relates to apparatus for continuously using a metalcleaning process of the molten alkali salt bath type, a preferred butnot necessarily the only example of which is the process knowncommercially as the Kl process and described in Patent No. 2,458,661'ofJanuary 11, 1949 to Hugh G. Webster and Clarence J. Falter.

Generally speaking, the method under consideration employs molten alkalisalts for cleaning metal strip continuously and includes the use oimeans for preventing directing and squeezeout rollers employed in suchapparatus from marring or scratching the cleaned surface or the surfacesbeing cleaned, and also means for preventing the deposit on such rollsof solid particles which would mar or scratch the steel strip.

It is understood that the term strip as used herein refers to a movingribbon of any desired width, not only ribbon under 12" wide and known tothe trade as strip, but also ribbon over 12 wide and known to the tradeas sheet, and also ribbon extremely narrow and known to the trade aswire.

It is also understood that while steel is the particular metal chosenfor descriptive purposes herein, and is more commonly used for treatmentby the process hereof, that copper may also be treated by the processhereof.

The process Patent No. 2,458,661 discloses a metal cleaning process ofthe molten alkali metal salt bath type wherein a first step is theimmersion oi the article being treated in a molten alkali salt bath atan operating temperature well above the melting point of the alkalimetal salt, such immersion operating to transform impurities into easilyremovable oxides. The second step in the process of that patent is theimmersion of the oxide coated article, coated with the oxide formed bythe rst step, into a bath which operates to remove the oxide coating.Such second bath may `be itself an alkali metal salt bath or it may be aweak acid bath.

The time of immersion of the article being cleaned in either of thesetwo baths is governed by operating conditions, which also govern thetemperatures of the bath. The immersion in the rst bath may be of anydesired period, in some cases being well below one minute, and in othercases being well in excess of one minute. The immersion inthe secondbath is generally quite brief, only enough to remove the readily re- 2moved oxides formed by the first bath. The immersion in the second bathis a variable, depending on the metal being treated.

The temperature of the rst bath is also as desired, ranging fromsomewhere above the melting point of the salt bath, to somewhere belowits vaporization point, or its decomposition point, Whichever is lower.Where the bath is of the preferred formula, as set forth in Patent No.2,458,661, the melting point is 550 F. approx., the decomposition pointis 1100o F. approx., and the vaporization point is 2500 F. approx., andthe bath temperature will be between 500 F; and 1100 F.

Between the rst and second baths there is customarily employed a waterrinse bath of any suitable type. This insures uniform oxide removal fromthe strip. It is well known that the presence of alkali (as from the rstbath) on a metal surface when immersed in an acid (as in the secondbath) tends to prevent oxide removal. The water rinse insures theabsence of alkali on the strip from the first bath as the strip entersthe second bath.

inasmuch as the process of the foregoing named patent may readily beunderstood upon reference to that patent and need not be furtherdescribed herein, reference to that process generally is here concludedwith the observation' that the preferred process hereof is identicalwith the process of that patent and with the understanding that thedisclosure of that patent is incorporated into this application byreference, to avoid the necessity of incorporating that disclosurhereinto expressly.

While the process of said Patent 2,458,661 is the one preferred for useas the process hereof, other processes and variations of said processmay also be used, to the extent disclosed herein.

The apparatus It is known to employ long troughs with directing rollersto treat strip continuously. Such apparatus is here used forcontinuously using the process above described on continuously movingstrip. However, certain improvements have herein been disclosed and willnow be described.

The improvements The improvements of this application relate to thetreatment of continuously moving strip. Such improvements are aimedIparticularly towards the utilization of rollers for directing andmoving strip continuously through the baths and to the use of means forpreventing the deposit on such rollers of solid particles which wouldmar the finish of the strip as it passes by and engages such rollers.

It can readily be understood to those skilled in the art that the use ofrollers for directing and moving strip continuously through molten saltbaths, water rinse baths, and oxide removing baths, is a prerequisite.rihis application specifically relates to the apparatus and the use ofsuch apparatus for preventing deposits on such rollers from marring thesurface of the strip engaging and passing by such rollers.

For an understanding of the apparatus and process hereof, and on theassumption that the reader hereof will have become by this time familiarwith the process of the aforesaid Patent 2,458,661, incorporatedhereinto by reference, refu erence may now be had to the followingspecication and detailed description to be read in connection with theaccompany drawing.

In this drawing:

Fig. 1 is a diagrammatic side view of a continuous strip treatingapparatus, including a furnace, a molten salt bath, a water rinse bath,and an ox ide removing bath.

Fig. 2 is a diagrammatic top plan view of the apparatus of Fig. l.

Referring now to the drawing, it will be ob served that the drawingshows at l a continuously moving strip being treated continuously by theprocess of the aforesaid patent.

The strip is first heated in a furnace 26 in order that its temperaturebe brought above the operating temperature of the molten salt bath,later to be described. Since the operating temperature of the moltensalt bath may range anywhere from 500 F. up to approximately 900 F.,depending upon operating conditions and the speed oi operation, with thelower end of the range being determined by the operating temperaturerequired for maintaining molten the salt bath, and with the maximumtemperature being determined by the temperature above which the saltbath vaporizes or decomposes, whichever is lower, it is generallycontemplated to heat the strip in the furnace to a temperature around1200 F. In some instances even higher temperatures, 1900 F.2200 F., areused. So heated the strip passes over a directing roller 2|.

It is pointed out here that the aforesaid heating is to a temperatureselected as proper for the particular alloy being treated and. is notcritical to the present invention.

In the event the steel strip leaving the furnace is at too high atemperature, it may be cooled by the use of steam sprayed out of thesteam pipes 22 just ahead of the roller 2 l It may be observed here thatif the steel is too hot as it enters the molten salt bath, it will causebubbling and splattering of the molten salt in the bath with undesirableeffects. To prevent this result, the steel strip may be cooled by thesteam from pipes 22. Air, water, or air-water-steam mixtures may also beused for cooling the strip in some cases.

In some instances the strip is cooler than the salt bath on entering it.For example, the strip might be entered at room temperature into thesalt bath.

The tubes 34 are merely cylindrical chambers immersed in the salt bath33 which house elongated ducts or pipes 15. Pipes 'i5 supply combustiblegas which is burned in a series of small burner ames disposed along thelength of a pipe 15 by forming a series of perforations 'H6 thereinthrough which the combustible gas is emitted and burned. The combustiongas is supplied from any suitable source through a pipe 'VI controlledin quantity by a valve i8 mounted thereon, and it is mixed with airpassed through a pipe 79 which joins pipes TI, both combustible gas andair mixing and passing into pipe 15, the air being in combustionsupporting quantity as controlled by a valve 80 mounted on pipe ll. TheWaste gases after combustion within tubes 34 are led away throughtubular ducts i6, each joining a tube Sil at right angles, and thence amanifold into duct 'il common to all which leads to stack 65 of the ventfor ultimate disposal ci the waste gases.

The molten salt bath Next in line to the directing roller 2l is a moltensalt bath which includes a tub 32 containing the molten salt 33, thelatter being of the molten alkali salt type. In the preferred process,it is generally of the formula of the aforesaid Patent 2,458,661,namely, i part by weight of alkali metal nitrate, 1.5-8.0 parts byweight of alkali metal hydroxide, and 0.1-0.5 part by weight of alkalimetal chloride.

The salt 33 is maintained molten by suitable heating means. One suitableheating means are the burning gas tubes 34 which are immersed in themolten salt.

Disposed on horizontal axes 35 above the normal level 36 of the moltensalt 33 are directing rollers 3?-38 which direct and move the strip lilfrom roller 2l down into the molten salt bath and then to a pair ofrollers 45, later to be described, which in turn direct the steel stripout of the bath 33 and towards rollers Q2, 33, and 44, d5, i6 whichdirect the strip under a water spray pipe 49 and through a water rinsebath 50 and an acid rinse bath 5i. The acid rinse is here disclosed asthe preferred means for removing the oxides formed in the molten saltbath 33, and is of the character described in the aforesaid Patent2,458,661, such acid rinse bath being a dilute acid selected from theclass consisting of hydrochloric and sulphurc acids, such that the bathwill react chemically with the oxidized coating of the steel strip toremove suchv coating and leave the strip free of the original impuritiesand of the oxidized coating as well. The immersion in the acid rinsebath is timed, however, to be such as not to expose at the surface otherimpurities not previously existent or not previously exposed and issufficiently brief so as not to permit the acid to attack the metal ofthe steel strip, but merely to remove the oxide coating formed by themolten salt bath itself.

While in the preferred embodiment the acid rinse bath is a dilute acidof the group comprising hydrochloric and sulphuric acids, it has beenfound, in some cases, practical to use other acids, such as nitric ornitric hydroluoric acids.

Thus far we have described nothing more than a continuous strip treatingprocess for carrying out the process of the aforesaid patent, such asmight be developed by anyone skilled in the art. i ]Now we turn to theimprovements of this applicaion.

1. First is the improvement in the nature of the rolls 37, 38, and 40.These rolls are not steel rolls, as are customarily contemplated inbaths used for treating continuously moving steel strip, but rather areof cast iron. It was discovered that when steel rolls were utilized, asthe rolls 31, 38, and 40, the steel rolls scratched the surface of thesteel strip I0, particularly objectionable if that steel strip waspolished stainless steel. It was further discovered that when cast ironwas used for the surface of such rolls, as by using a cast iron shell ona steel roll, there was formed in the surfaces of the cast iron rolls,minute asesinas 5 fissures produced by the removal of the graphiticcarbon from the cast iron rollsdue to the action of theA molten salt on.the cast iron rolls. These minute fissures provided excellent reservoirsor pockets for the huid or molten salt on the rolls and. the molten saltwas observed to function as a lubricant on the ferritic surface of thecast iron rolls, the nssured rolls functioning in. a mannerl similar toa porous metal roll'. The molten salt clinging to the surfaces of thesezrolls acts as an excellent lubricant and prevents the rolls fromscratching the polished: stainless steel strip passing by and'. engagingthem.

Similarly, the rolls; 4-0 for squeezing off excess molten salt from thestripi t leaving the salt bath; were also made of castv irorL While theuse oit cast iron for the rollers is of special value because of theeffect on such rollers of the molten salt bath of Patent 2,458,661, itmay here be pointed. out that the same beneficial results. may beobtained with other salt baths, provided they be of a class or nature asto remove graphitic carbon from. cast iron. The latter is the essentialor determining factor in the cooperative relationship of the bath andthe rollers, cooperating to the end that the rollers will not scratchthe strip'.

2. However, it Was found necessary to provide means for heating therolls 40 so as to maintain the salt at such rolls in a molten and fluidcondition. Obviously, any suitable heating means could be employed.However, it was observed that by providing a rather close fittinginsulating cover 60 over the tub 32, the heat radiated from the surfaceof the molten salt 33, heated by the heating means 34 necessary tomaintain such salt molten, operated to maintain the salt molten, even atthe rolls 40, and the molten salt on such rolls acted as a lubricant toprevent scratching of the polished stainless steel strip by the rolls40.

In addition, as long as the salt was maintained molten at the rolls 40,the formation and deposit of alkali carbonates and other crystallinematerials on the strip I6 at the rolls 40, and on the rolls themselves,was inhibited and, thus, another cause for mari'ing of the strip waseliminated. it was discovered that unless the salt was maintained moltenat the rolls 40, then solid carbonates and other crystalline materialswere deposited on such rolls and these caused the scratching of thestrip. The provision of the insulating hood 60 operated in the mannerabove described to prevent the formation of these solid deposits on therolls 40 and, thus, prevented marring of the strip due to the presenceof such solids.

3. Still another improvement is the venting of the water rinse bath asby the provision of a closely fitting venting hood 64 connected to anexhaust fan or the like to outlet at 65. It was observed that in theabsence of any vent, such as the vent 64, alkali vapors from the surfaceof the molten salt, particularly in the water rinse bath 50, condensed,with the condensate settling on the surface of the finished strippassing through such rinse bath in or near such rinse bath and causingsurface imperfections. The provision of the vent 64 prevented suchalkali vapors from condensing and coming into contact with the strip andsettling on the strip and preventing the marring that would otherwise bethe case. Not only does the vent 64 prevent spotting due to thecondensed vapors but also vent 64 accelerates removal of the vaporsthemselves. These vapors, coming into contact with the strip,

6 mayl cause discoloration.4 Their rapidi removal., by vent S4, inhibitssuch action.

4.- Sti-ll anotherv improvement is theA location of the Water rinsetank; This tank. is located as close to the molten salt bath aspossible.. It been discovered that improved resul-ts, with respect tothe preventing of imperfections and discolorations on the stainlesssteel stri-p,` may be obtained by decreasing the distance' between thepoint where the strip leavesl the molten salt bath and the point whereit enters the water' rinse bath.

By providing the water rinse bath as close to the molten salt bath aspossible, it becomes possible to rinse the strip at the highest possibletemperature, and this insures superior rinsing with less facilities; Thequench action of the rinse is more rapid than otherwise, and this aidsin loosening the oxide at the surface and facilitates the* removal ofthe oxide in the weak acid bath 5I.

It i-'snoted that the strip is cooled before it reaches roller 42, whichis rubber covered. This is accomplished by passing the stri-pA throughthe bath 50' before it reaches rol-Ier d2, or by providing water spray'means ahead of roller 421 5. Still another improvement is in theprovision of means for preventing' the products of combustion of theburners '34 from coming in contact with the surface of the salt bath 33.The outlets of the burners 34 are connected through outlet pipes 10 anda manifold 'H to the vent 64, so as to be exhausted without coming incontact with the molten salt bath 36.

It has been discovered that if the products of combustion in the burners3d come in contact with the surface of the salt bath 33, there areformed carbonates which deposit on the rolls 3l' and 38 and causemarring of the surface of the strip. The provision of the outlets 10 and'Il for these products of combustion eliminates such carbonates andeliminates this cause for marring the strip.

6, It is noted that the salt bath is as close to the furnace as possibleso as to reduce the heat loss in the strip as it passes from the furnaceto the salt bath, and thus reduces the cost of fuel for maintaining boththe strip and the salt bath at the operating temperature for the bath,Whatever that temperature is.

It is also noted that the salt bath, generally below 900 F. intemperature, functions as a quench for the higher temperature steelstrip and is located close enough to the exit end of furnace 20 as toreceive the strip within two minutes from the time the strip leaves suchfurnace. Thus it functions to prevent carbide precipitation, when thestrip is of stainless steel of the nickel chrome type. Suchprecipitation, an undesirable phenomenon, occurs when strip of stainlesssteel of the nickel chrome type is left at 900 F. or above for more thantwo minutes. The quench action of the salt bath oc-curs within twominutes because the bath is located so close to the furnace and preventssuch carbide precipitation.

Summary We have here disclosed apparatus for producing cleaned andscratch-free surfaces on -continuously moving strip, such as polishedstainless steel strip. The process herein disclosed is the K1 processdescribed in the aforesaid Patent 2,458,661. The apparatus hereofcontains certain improvements listed as follows:

(1) The use of cast iron surfaces for the holddown, directing, andsqueeze-out rollers in the molten salt bath.

(2) The use of an insulated cover or hood for the salt bath at thesqueeze-out rollers to maintain the molten salt fluid at su-ch rollers.

(3) The use of a shield and ventilator for the water rinse immediatelyfollowing the molten salt bath.

(4) The outletting of the products of combustion of the burners used forheating the molten salt bath away from the salt bath so as to preventthese products from coming into contact with the salt bath and formingcarbonates and the like to deposit on the rollers and cause scratchingof the strip.

(5) The locating of the salt bath as close as possible on the one handto the strip heating furnace, and on the other hand to the water rinsefollowing the salt bath.

Now having described the invention of this application and theconstru-ction shown in the appended drawing, reference should now be hadto the cla-im which follows.

In the cleaning of metal strips by running continuous lengths thereofthrough a hot molten alkali metal salt bath reactive With the surface ofthe metal and impurities therein whereby the strip of metal emerges fromsaid bath in a highly heated state having evenly etched surfaces coatedby a thin Water soluble iilm of said molten salt, the method comprisingquenching and rinsing said hot molten salt coated lm on said metal stripby passing the same continuously into a cold water bath whereinsubstantial uneven ebullition and vaporization of vapors and alkalinechemicals take place upon contact with said bath to form a vaporous mistabove said water bath from which alkaline vapors and condensed dropletstherefrom tend to redeposit irregularly upon the strip to spot the sameand impair the evenly etched surface formed in the molten alkali metalsalt bath, and simultaneously and continuously withdrawing all of thesaid vaporous mist and mechanically entrained alkaline chemicals fromover the entire surface of said bath to prevent redeposition of alkalinedroplets upon said cleaned metal strip.

RALPH DUNLEVY.

HAROLD FRICK.

JOHN H. SHOEMAKER,

References Cited in the le of this patent UNITED STATES PATENTS NumberName Date 2,212,588 Csanyi Aug. 27, 194() 2,311,099 Tainton Feb. 16,1943 2,311,139 Tainton Feb. 16, 1943 2,458,661 Webster Jan. 11, 1949

